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University of Ottawa School of Information Technology and Engineering

(SITE)

Introductory Electronics Laboratory

Prepared for

Engineering Summer School Summer 2005

© Salim Y. Said

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Morning Session

Purpose

The purpose of this session is to get familiar with the electronics lab. We will discover resistors, capacitors, and operational amplifiers. We will also learn how to use the breadboard, power supplies, multi-meters, function generator, and the oscilloscope.

Resistors

A resistor is a two-terminal electrical component that creates an electrical potential difference across its terminals that is proportional to the current passing through it. The ratio of potential difference (also called voltage) to current is known as its electrical resistance (or simply resistance). In SI units, the resistance is measured in ohms (O).

Figure 1: A Resistor

Colour 1st Band 2nd Band 3rd Band (multiplier)

4th Band (tolerance)

Black 0 0 x 100 Brown 1 1 x 101 ± 1%

Red 2 2 x 102 ± 2% Orange 3 3 x 103 Yellow 4 4 x 104 Green 5 5 x 105 ± 0.5% Blue 6 6 x 106 ± 0.25%

Violet 7 7 x 107 ± 0.1% Gray 8 8 x 108 ± 0.05% White 9 9 x 109 Gold x 0.1 ± 5% Silver x 0.01 ± 10%

Question: Can you read the value of the resistor in figure 1?

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Capacitors

The capacitor's capacitance (C) is a measure of the amount of charge (Q) stored on each plate for a given potential difference or voltage (V) which appears across the plates. In SI units, a capacitor has a capacitance is measured in farad (F).

Figure 2: different capacitor models

Operational Amplifiers

An amplifier is a device that uses a small amount of energy to control a larger amount. The relationship of the input to the output--usually expressed as a function of the input frequency--is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain. Amplifiers are typically utilized over a specific range of frequencies and are generally of maximal utility if the gain is constant in that range.

Figure 3: 741 Operational Amplifier

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Breadboard

A breadboard is a material or a device used to build a prototype of an electronic circuit.

Figure 4: Breadboard

The breadboard has many strips of metal (copper usually) which run underneath the board. The metal strips are laid out as shown in figure 5.

Figure 5: Breadboard Metallic Strips

These strips connect the holes on the top of the board. This makes it easy to connect components together to build circuits. To use the bread board, the legs of components are placed in the holes (the sockets). The holes are made so that they will hold the component in place. Each hole is connected to one of the metal strips running underneath the board. The long top and bottom row of holes are usually used for power supply connections.

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Power Supply

A power supply is a separate unit or part of a circuit that supplies power to the rest of the circuit or to a system. The power supply takes the current from your wall electrical socket and coverts it into the various voltages your circuit needs.

Figure 6: Power Supply

Multimeter

A meter is a measuring instrument. An ammeter measures current, a voltmeter measures the potential difference (voltage) between two points, and an ohmmeter measures resistance. A multimeter combines these functions and possibly some additional ones as well, into a single instrument.

Figure 7: Multimeter

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Before you turn the power supply, let the lab demonstrator check your circuit. Not doing so, may cause damage to the components or equipment!!

Practice 1

Build up the circuit as shown in figure 8. R1 = O and R2 = O. Measure using the multi-meter the values of the current I, and the voltages across R1 and R2 respectively.

• Is V = V1 + V2 ?

Figure 8: Resistors in Series

Build up the circuit as shown in figure 9. R1 = O and R2 = O. Measure using the multi-meter the values of the current I, and the voltages across R1 and R2 respectively. Also measure the currents following through the resistors, I1 and I2.

• Is I = I1 + I2 ? • Can you find a relationship between V, V1, and V2?

Figure 9: Resistors in Parallel

+

- Vs

5 V

R1

I

+ V1

-

R2

+ V2 -

+

- Vs

5 V

I1

R1

+

V1

-

I2

R2

+

V2

-

I

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In order to perform the practice exercise #2, you need to get familiar with use of the function generator and the oscilloscope. Please listen to the lab demonstrator instructions and explanations before performing the exercise.

Practice 2

Set the function generator to produce a 5 kHz sine wave with amplitude of 2 volts (one peak only). Connect the output of the function generator to the inputs of the oscilloscope. Tune the oscilloscope so that you may clearly see the waveform. Identify the peak-to-peak amplitude, the period, and the frequency of the waveform. Change the waveform shape to triangle and square waves.

Function Generator

A function generator is a device that can produce various patterns of voltage at a variety of frequencies and amplitudes. It is used to test the response of circuits to common input signals. The electrical leads from the device are attached to the ground and signal input terminals of the device under test.

Figure 10: Simple Function Generator

Most function generators allow the user to choose the shape of the output from a small number of options.

• Square wave - The signal goes directly from high to low voltage. • Sine wave - The signal curves like a sinusoid from high to low voltage. • Triangle wave - The signal goes from high to low voltage at a fixed rate.

The amplitude control on a function generator varies the voltage difference between the high and low voltage of the output signal. The frequency control of a function generator controls the rate at which output signal oscillates.

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Oscilloscope >>> http://www.doctronics.co.uk/scope.htm#screen

An oscilloscope is easily the most useful instrument available for testing circuits because it allows you to see the signals at different points in the circuit. The best way of investigating an electronic system is to monitor signals at the input and output of each system block, checking that each block is operating as expected and is correctly linked to the next. With a little practice, you will be able to find and correct faults quickly and accurately.

Figure 11: Oscilloscope

As you can see, the screen of this oscilloscope has 8 squares or divisions on the vertical axis, and 10 squares or divisions on the horizontal axis. Usually, these squares are 1 cm in each direction:

Figure 12: Oscilloscope Display

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Afternoon Session Objective

The objective of this session is to use all the knowledge and skills learned in the morning session to build a signal generator than can produce a square wave using 741 operational amplifiers.

Square Wave Generator

One simple and inexpensive circuit used to generate a square-wave is the relaxation oscillator circuit. It is also known as the “a stable multi-vibrator”.

Figure 13: Simple Square wave Generator Circuit

Before you turn the power supply, let the lab demonstrator check your circuit. Not doing so, may cause damage to the components or equipment!! Connect the circuit shown in figure 13, with the following resistor/capacitor values:

• R1 = 10 K O • R2 = 12 K O • Rf = 500K O to 50O • C = 0.1µF

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Note that the value of Vcc = 15V and Vin = 0V (grounded).

Before you turn the power supply, let the lab demonstrator check your circuit. Not doing so, may cause damage to the components or equipment!! Using the oscilloscope, display Vo on channel 1 screen. Record the peak-to-peak amplitude, the period, and the frequency of the waveform.

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Circuit Analysis

• Time constant τ = Rf C

• Vn = Vcc 21

1

RRR+

.

Let Vo = Vcc. Vc will increase toward positive Vcc with a time constant τ. When Vc exceeds Vn, Vo will change to negative Vcc. In other words, the amplifier will flip from saturating in the positive direction to saturating in the negative direction. In a similar manner, Vc will charge toward negative Vcc until it falls below -Vn. As a result of this positive and negative charging, a square wave will be generated varying between ± Vcc. Consider now the analysis of Vc. The waveform of Vc is that of ± Vn exponential charging saw tooth. Consider the half period as Vc charges from –Vn to + Vn. Vc = -Vn + (Vcc – (-Vn))(1 – e –t / R

f C )

When Vc = Vn, t= T /2 è Vn = -Vn + Vcc + Vn – (Vcc + Vn)e –T / 2RC

è ncc

ncc

VVVV

+−

= e –T / 2RC è T = 2RC ln

+−

ncc

ncc

VVVV

But Vn = Vcc 21

1

RRR+

è T = 2RC ln

+

2

121

RR

è f = T1

= 1

2

1

R2R

1ln 2RC−

+

The following are the corresponding waveforms of the generated square wave and the Vn exponential saw tooth curves.

+V

-

t

+V

-

VOLTS

T/2